const age: number = 18;
const username: string = "jack";
const isMan: boolean = true;
const u: undefined = undefined;
const n: null = null;
const s: symbol = Symbol("s");
let a: any = 1;
a = "1";
a = true;
a = null;

const arr: number[] = [1, 2, 3];
// const arr2: Array<number> = [1, 2, 3];
const t: [number, string] = [1, "2"];
const coord: [number, number] = [100, 20];

type User = {
  name: string;
  age: number;
};
const user: User = {
    name: "jack",
    age: 18, 
}

function add(a: number, b: number): number {
    return a + b;
}

function sayHello(): void {
    console.log("hello");
}